Use of a calcium potassium nitrate salt for the manufacture of a heat transfer fluid

ABSTRACT

The present invention relates to the use of a double salt of Ca(NO 3 ) 2  and KNO 3  for the manufacture of a melt, in particular a nitrate-based heat transfer fluid (HTF) and/or thermal energy storage fluid, for example in solar energy applications, such as in solar electrical power plant systems using a parabolic through, a central receiver or a linear Fresnel, which have both a low melting temperature and a high decomposition temperature, as well as to methods for the manufacture thereof. Using said double salt, a melt could be manufactured comprising at least NaNO 3 , KNO 3  and Ca(NO 3 ) 2 , preferably, an eutectic ternary melt comprising Ca(NO 3 ) 2 , NaNO 3  and KNO 3  in a weight ratio of about 42:15:43 with an operating temperature of about 131° C. to about 560° C.

FIELD OF THE INVENTION

The present invention relates to the use of a calcium potassium nitratesalt for the manufacture of a melt, in particulate a nitrate-based heattransfer fluid (HTF) and/or thermal energy storage fluid, for example insolar energy applications, such as in solar electrical power plantsystems using a parabolic through, a central receiver or a linearFresnel, which have both a low melting temperature and a highdecomposition temperature, as well as to a method for the manufacturethereof.

BACKGROUND OF THE INVENTION

Concentrating solar power (CSP) uses mirrors to focus solar energy toboil water and to make high pressure steam. The steam subsequentlydrives a turbine and generator unit to generate electricity. There is aneed to bring CSP electricity cost down to the point of beingcompetitive with traditional fossil fuel-based electricity. An advancedlow melting point heat transfer fluid (HTF) with a high thermalstability is a key technical advance necessary to reduce the cost of CSPelectricity. Such a material would enable higher temperature operationand increased efficiency in converting solar energy to electricity.Increasing the maximum fluid output temperature of current CSP plantsfrom 390° C. to 500° C. would increase the conversion efficiency of theRankine power block, thereby reducing the levelized energy cost by 2cents/kWh. Achieving 500° C. operation would also double theeffectiveness of sensible heat thermal storage systems, significantlyreducing the capital cost of thermal storage (Justin W. Raade and DavidPadowitz, Development of Molten Salt Heat Transfer Fluid With LowMelting Point and High Thermal Stability, J. Sol. Energy Eng. 133,031013 (2011)).

Furthermore, an advanced low melting point heat transfer fluid couldalso be used as a thermal energy storage fluids in solar energyapplications. Heat storage allows a solar thermal plant to produceelectricity at night and on overcast days. This allows the use of solarpower for continuous power generation as well as peak power generation,with the potential of displacing both coal- and natural gas-fired powerplants. Additionally, the utilization of the generator is higher whichreduces cost.

Heat is transferred to a thermal storage medium in an insulatedreservoir during the day, and withdrawn for power generation at night.The envisioned product should be cheap to produce, easy to make and easyto handle.

A number of different solutions to the aforementioned problem has beenproposed, one of them being the use of nitrate-based salts used as amelt (molten salt). Molten salts exhibit many desirable heat transferqualities at high temperatures. They have high density, high heatcapacity, high thermal stability, and very low vapour pressure even atelevated temperatures. Their viscosity is low enough for sufficientpumpability at high temperatures, and many are compatible with commonstainless steels. Salts of many varieties are currently available inlarge commercial quantities from several suppliers.

Most commonly, a binary salt based on NaNO₃ and KNO₃ and commonly knownas Solar Salt, is used as heat transfer fluids and as thermal energystorage fluids. The Solar Salt consists of the eutectic mixture of 60%NaNO₃ and 40% of KNO₃. NaNO₃ melts at 307° C. and KNO₃ melts at 337° C.The mixture at its eutectic point exhibits a drastically reduced meltingpoint of 222° C. This represents a melting point suppression of 85° C.from the lowest melting single component. It is produced as a doublesalt in solid (granular) form, for example as disclosed in U.S. Pat. No.4,430,241 (Fiorucci, 1984). Although the salt is very cheap and has ahigh thermal stability, a major drawback of this salt is its highmelting temperature (220° C.).

Bradshaw et al. in Solar Energy Materials 21 (1990) 51-60 describe theuse of ternary nitrate salts comprising nitrate salts of Na, Ca and K,for solar thermal energy systems (see Tablel therein). Adding Ca(NO₃)₂to the Solar Salt showed to lower the melting temperature, which is anadvantage as it lowers the risk of solidifying of the salt mixture inthe system, blocking pumps, piping, etc.

As is commonly known, Ca(NO₃)₂ in its anhydrous form is a hygroscopicsolid, forming the (liquid) tetrahydrate salt Ca(NO₃)₂.4H₂O with amelting point of about 43° C. It is commercially available as a liquidsolution or in solid particulate form, where it is mixed with ammoniumnitrate to reduce its tendency to absorb water, in particulate waterfrom the air. In its particulate form, it is difficult to handle.

From the disclosed melt compositions in Bradshaw et al. in Solar EnergyMaterials 21 (1990) 51-60, it can be concluded that the thermalstability decreases with increasing amounts of Ca(NO₃)₂. At amounts of42 weight % of Ca(NO₃)₂, the decomposition temperature is about 500° C.and a solid phase (CaCO₃) was visually detected. Similar research wasconducted by the US Department of Energy and the Sandia NationalLaboratories and several reports are available on the internet (see e.g.Steven St. Laurent, Thermocline Thermal Storage Test for Large-scaleSolar Thermal Power Plants), where a melt mixture is disclosedmanufactured from melting together 30 weight % of Ca(NO₃)₂, 24 weight %of NaNO₃, and 46% of KNO₃, all in solid form.

Hitec XL (Coastal Chemical) is commercially available as an aqueoussolution of a ternary nitrate salt mixture containing 59 weight % water,comprising Ca(NO₃)₂, NaNO₃ and KNO₃, of which different compositionshave been reported. When the water is boiled off, the melt has acomposition which has been reported to be 15% NaNO₃, 43% KNO₃ and 42%Ca(NO₃)₂ (Kelly et al., 2007), whereas Kearney et al (2003) quote 7%NaNO₃, 45% KNO₃ and 48% Ca(NO₃)₂. The eutectic mixture lies at aconcentration ratio of 7%/30%/63% (ENEA, 2001). In practice, and mainlyfor cost reasons, the exact eutectic concentration is not employed asthe solidification temperature is not very sensitive to the exact mixingratio (Large-Scale Solar Thermal Power, Werner Vogel and Henry Kalb,Wiley-VCH Verlag GmbH & Co, KGaA, Weinheim, 2010, page 245. The Hitec XLmixture is manufactured by dissolving the three salts (Ca(NO₃)₂, NaNO₃and KNO₃) in water. It has the disadvantage that, in order to obtain theeutectic melt, this high amount of water (59 weight %) needs to beboiled off, leading a large energy consumption.

U.S. Pat. No. 7,588,694 B1 (Bradshaw et al., 2009) describes the use ofquaternary compositions comprising nitrate salts of Na, K, Li and Ca.For comparison, an eutectic nitrate salt composition is disclosedcontaining 21 mol % Na, 49 mol % K, and 30 mol % Ca, having a meltingtemperature of 133° C. (Table 2). No decomposition temperature is given.Also, for comparison, an eutectic nitrate salt composition is disclosedcontaining 30 mol % Na, 50 mol % K, and 20 mol % Ca, having a meltingtemperature of 505° C. (Table 3). No melting temperature is given. Theuse of lithium nitrate is undesirable due to the high cost thereof.

There is little or no research on higher order mixtures of nitrates.

Hence, there is a need for a low-cost nitrate salt based mixture thathas both a low melting temperature and a high decomposition temperature,and is easy to make and to handle.

DETAILED DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide a nitrate saltcomposition for use as heat transfer fluid (HTF) and as thermal energystorage fluids which have both a low melting temperature and a highdecomposition temperature, as well as to a method for the productionthereof, and which are easy to make and to handle.

This object is met by the double salt of Ca(NO₃)₂ and KNO₃ of thepresent invention according to the independent claim 1.

The inventors have realized that the use of a double salt comprisingCa(NO3)₂ and KNO₃, hereafter called Ca/K double salt, has uniqueproperties which allows it to be used easily for the manufacture of anitrate-based heat transfer fluid (HTF), in particular to produce a meltcomprising NaNO₃, KNO₃ and Ca(NO₃)₂.

According to one embodiment, the Ca/K double salt is a particulateproduct from a melt, comprising 1.5 to 5.5 weight % of K (present asKNO₃), 70 to 80 weight % of Ca(NO₃)₂ and 13 to 18 weight % of water.

According to another embodiment, the Ca/K double salt is a particulateproduct from a melt, comprising 2.5 to 4.0 weight % of K (present asKNO₃), 74 to 75 weight % of Ca(NO₃)₂ and 15 to 16 weight % of water.

According to one embodiment, the double salt of Ca(NO₃)₂ and KNO₃ isammonium-free.

According to a preferred embodiment, the particulate product is aproduct marketed under the tradename “NitCal/K” by Yara International,

Oslo, Norway. NitCal/K is a Ca/K nitrate with the general formulaKNO₃.5Ca(NO₃)₂.10H₂O and having an average chemical composition of about9 weight % of KNO₃ (about 3.5 weight % of K), about 74 weight % ofCa(NO₃)₂ and about 16 weight % of water. It can be manufactured as afree-flowing solid particulate product with a low water absorption andcaking tendency.

Although the different components may be provided as a combined aqueoussolution, such as the Hitec XL solution, or as different aqueoussolutions of Ca(NO₃)₂, KNO₃ and NaNO₃ , the advantage of the presentinvention is that a melt can be produced based on mixing solidcomponents only, followed by controlled heating, i.e. heating up thesolid mixture using a specific temperature program and/or temperaturegradient. This process avoids removal of the water from the aqueoussolution according to the prior art.

According to one embodiment, a melt comprising at least NaNO₃, KNO₃ andCa(NO₃)₂ is produced, comprising mixing NaNO₃, KNO₃, and the Ca/K doublesalt in their particulate form according to the invention, followed bycontrolled heating and melting of the resulting mixture. The skilledperson will understand that several other options are available formixing and melting the three components, such as mixing two out of threecomponents and adding a third component to the melt.

According to one embodiment, the Ca/K double salt is used to directlyproduce an HTF (i.e. a melt), in particular to produce an HTF (i.e. amelt) comprising NaNO₃, KNO₃ and Ca(NO₃)₂. When the Ca/K double saltused as a source of Ca, this avoids boiling of large amounts of water,as is the case of the Hitex XL product. Within the context of thisinvention, with “directly” is meant a use which does not involve theproduction of an intermediate solid product comprising NaNO₃, KNO₃ andCa(NO₃)₂.

According to one embodiment, the melt comprises Ca(NO₃)₂, NaNO₃ and KNO₃in a weight ratio of (30 to 50):(10 to 20):(30 to 50), provided the sumof the weight of Ca(NO₃)₂, NaNO₃ and KNO₃ is 100%.

According to a preferred embodiment, the melt comprises Ca(NO₃)₂, NaNO₃and KNO₃ in a weight ratio of about 42:15:43, provided the sum of theweight of Ca(NO₃)₂, NaNO₃ and KNO₃ is 100%. The latter ratio is aboutthe eutectic point of the ternary composition and offers the lowestmelting point. Hence, it is advantageous to mix the three components ina solid particulate form and melt the resulting particulate mixture, asless energy will be required.

With the composition according to the invention, a melting temperatureof about 131° C. is achieved, which is advantageously lower thatachieved using the currently used binary salt (220 C.) and is in therange of melting temperatures disclosed in the prior art for this kindof ternary salt.

With the composition according to the invention, a decompositiontemperature of 536° C. is achieved, which is surprisingly in the samerange, and even higher, than the decomposition temperature of thecurrently used Solar Salt (525° C.) and higher than the reporteddecomposition temperature of the HitecXL product (500° C.). Verysurprisingly, using the Nitcal/K product as described above in admixturewith NaNO₃ and KNO₃ salts of a superior quality (i.e. containing fewimpurities), the resulting melt having a weight ratio of about 42:15:43can reach a decomposition temperature of 569° C. (at 3% weight loss),which is surprisingly high, such that an upper operating temperature ofthe melt of about 550 to 560° C. is achievable. Using NaNO₃ and KNO₃salts of a lower quality (i.e. containing more impurities), adecomposition temperature of 525° C. (at 3.5 % weight loss) is obtained,which corresponds to an upper operating temperature of the melt of about510 to 530° C. Without being bound by theory, it is assumed that thesehigh upper operating temperatures are achievable due to the high qualityof the Nitcal/K product, i.e. its low level of impurities. Hence, usingthe Nitcal/K product as described above, in admixture with NaNO₃ andKNO₃ salts of a high quality (i.e. containing few impurities), theresulting melt having a weight ratio of about 42:15:43 has a wideoperating temperature of about 131° C. to about 560° C. A widetemperature range is advantageous as less salt is needed to absorb thesame amount of energy.

Furthermore, when using the specific Nitcal/K product for manufacturingthe HTF melt, which is chloride-free, the HTF melt is much lesscorrosive than other existing HTF melts, which means that the lifetimeof system components in contact with the HTF melt is extended.

The Ca/K double salt may be produced according to a method disclosed inU.S. Pat. No. 6,610,267 (Norsk Hydro, 2003), which document is enclosedherein by reference. In summary, a melt is produced by mixing apotassium source with a calcium nitrate source and heating the mixtureto a temperature of 150 to 155° C. An aqueous Ca(NO₃)₂ solution issuitable as a calcium nitrate source. Potassium nitrate (KNO₃) in asolid or aqueous form is suitable as a potassium source, butalternatively also KOH, neutralised with nitric acid can be used as apotassium source. Prior to conventional particulation, the water contentin the melt is adjusted by evaporation. Furthermore, it was found thatundercooling was avoided and that particulation with conventionalmethods (such as prilling and granulation) was possible when the melthas a certain concentration ratio of K, Ca(NO₃)₂ and water, inparticular comprising 1.5 to 5.5 weight % of K (present as KNO₃), 70 to80 weight % of Ca(NO₃)₂ and 13 to 18 weight % of water. Granulationprovided particles with a nice spherical shape, and the crushingstrength of 2.8 mm particles was 3 to 5 kg. No caking or post-meltreactions occurred during cooling of the material.

The invention also relates to a method for the manufacture of a melt asdisclosed in the application.

According to one embodiment, the invention relates to the manufacture ofa nitrate-based heat transfer fluid (HTF), comprising mixing a doublesalt of Ca(NO₃)₂ and KNO₃, and at least NaNO₃ and KNO₃, followed bycontrolled heating and melting of the resulting mixture. The melt may beused directly as an HTF or as a thermal energy storage fluid.

According to one embodiment, the invention relates to the manufacture ofa nitrate-based melt, preferably to be used as heat transfer fluid (HTF)and/or a thermal storage fluid, comprising mixing a double salt ofCa(NO₃)₂ and KNO₃, with NaNO₃ and KNO₃.

According to another embodiment, the invention relates to themanufacture of a nitrate-based melt, preferably to be used as heattransfer fluid (HTF) and/or a thermal storage fluid, comprising mixing adouble salt of Ca(NO₃)₂ and KNO₃, with NaNO₃ and KNO₃, wherein 56 weight% of the double salt of Ca(NO₃)₂ and KNO₃, 15 weight % of NaNO₃ and 38weight % of KNO₃ are mixed, followed by controlled heating and meltingof the resulting mixture, such that the resulting melt comprisesCa(NO₃)₂, NaNO₃ and KNO₃ in a weight ratio of about 42:15:43. Such amethod provides a (nearly) eutectic melt having a low melting point anda high decomposition point.

According to a preferred embodiment, the double salt of Ca(NO₃)₂ andKNO₃, NaNO₃ and KNO₃ are mixed in a solid particulate form . This hasthe advantage that said components of the melt may be used onconventional systems which are designed and are being used for theproduction of the binary Solar Salt, without large investments.

The invention will now be illustrated by an example. Such example shouldnot be construed as limiting the scope of the invention, which isdefined by the appended claims.

DESCRIPTION OF FIGURES

FIG. 1: Differential Scanning calorimetric (DSC) diagram of the ternarysalt mixture consisting of 42 weight % of Ca(NO₃)₂, 15 weight % of NaNO₃and 43 weight % of KNO₃.

FIG. 2: Thermogravimetric analysis (TGA) of the ternary salt meltconsisting of 42 weight % of Ca(NO₃)₂, 15 weight % of NaNO₃ and 43weight % of KNO₃.

EXAMPLES Example 1

To obtain an eutectic ternary melt consisting of 42 weight % ofCa(NO₃)₂, 15 weight % of NaNO₃ and 43 weight % of KNO₃, 15 weight % ofparticulate NaNO₃ (technical grade), 38 weight % of particulate KNO₃(technical grade) and 56 weight % of particulate Nitcal/K (YaraInternational SA, Oslo) was mixed in a laboratory scale mixer and heatedto about 131° C., at which temperature the melting of the mixturestarted. All the water was evaporated at a temperature of about 250° C.The composition of Nitcal/K is 9.2 weight % KNO₃, 74.5 weight % Ca(NO₃)₂and 16.3 weight % of water.

A DSC diagram of the ternary mixture is shown in FIG. 1 showing themelting behaviour of the mixture.

A thermogravimetric analysis (TGA) of the ternary salt melt is shown inFIG. 2, in comparison with the commonly used binary melt (Solar Salt).It shows a decomposition temperature of 525° C. at a 3.5 weight loss,compared to a decomposition temperature of 525° C. at a 2.8 weight lossfor the binary melt.

1-20. (canceled)
 21. Use of a double salt of Ca(NO₃)₂ and KNO₃ for themanufacture of a nitrate-based heat transfer fluid (HTF) or a nitratebased thermal energy storage fluid, wherein the double salt of Ca(NO₃)₂and KNO₃ is a particulate product from a melt comprising 1.5 to 5.5weight % of K (present as KNO₃), 70 to 80 weight % of Ca(NO₃)₂ and 13 to18 weight % of water.
 22. The use according to claim 21 , wherein thedouble salt of Ca(NO₃)₂ and KNO₃ is a particulate product from a meltcomprising 2.5 to 4.0 weight % of K (present as KNO₃), 74 to 75 weight %of Ca(NO₃)₂ and 15 to 16 weight % of water.
 23. The use according toclaim 21, wherein the double salt of Ca(NO₃)₂ and KNO₃ has the generalformula KNO₃.5Ca(NO₃)₂.10H₂O and has an average chemical composition ofabout 9 weight % of KNO₃ (about 3.5 weight % of K), about 74 weight % ofCa(NO₃)₂ and about 16 weight % of water.
 24. The use according to claim21, wherein the double salt of Ca(NO₃)₂ and KNO₃ is ammonium-free. 25.The use according to claim 21, wherein the melt comprises at leastNaNO₃, KNO₃ and Ca(NO₃)₂.
 26. The use according to claim 21, wherein themelt comprises Ca(NO₃)₂, NaNO₃ and KNO₃ in a weight ratio of (30 to50):(10 to 20):(30 to 50), provided the sum of the weight of Ca(NO₃)₂,NaNO₃ and KNO₃ is 100%.
 27. The use according to claim 21, wherein themelt comprises Ca(NO₃)₂, NaNO₃ and KNO₃ in a weight ratio of about42:15:43.
 28. The use according to claim 21, wherein a melt comprisingNaNO₃, KNO₃ and Ca(NO₃)₂ is produced, comprising mixing NaNO₃, KNO₃ andthe double salt of KNO₃ and Ca(NO₃)₂ in particulate form, followed bycontrolled heating and melting of the resulting mixture.
 29. The useaccording to claim 21, wherein the melt has a melting temperature of131° C.
 30. The use according to claim 21, wherein the melt has adecomposition temperature of at least 510° C., preferably at least 525°C., more preferably at least 550° C.
 31. Method for the manufacture of anitrate-based melt, comprising mixing a double salt of Ca(NO₃)₂ andKNO₃, with NaNO₃ and KNO₃, followed by controlled heating and melting ofthe resulting mixture.
 32. The method according to claim 31, wherein 56weight % of a double salt of Ca(NO₃)₂ and KNO₃, 15 weight % of NaNO₃ and38 weight % of KNO₃ are mixed, followed by controlled heating andmelting of the resulting mixture, such that the resulting melt comprisesCa(NO₃)₂, NaNO₃ and KNO₃ in a weight ratio of about 42:15:43.
 33. Themethod according to claim 31, wherein the double salt of Ca(NO₃)₂ andKNO₃, NaNO₃ and KNO₃ are mixed in a solid particulate form. 34.Particulate mixture comprising a mixture of a double salt of Ca(NO₃)₂and KNO₃, with NaNO₃ and KNO₃ mixed in a solid particulate form. 35.Particulate mixture of claim 34, comprising 56 weight % of the doublesalt of Ca(NO₃)₂ and KNO₃, 15 weight % of NaNO₃ and 38 weight % of KNO₃.
 36. Use according to claim 21, for use in solar energy applications.